media: staging: imgu: Remove redundant checks

Remove redundant checks for less than zero on unsigned variables.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Rajmohan Mani <rajmohan.mani@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>